1. Field of the Invention
The present invention relates to a fuel cell generation apparatus and a method for starting the same, the fuel cell generation apparatus being composed of a plurality of parallelly connected fuel cells and used as a large capacity generation facility.
2. Description of Related Art
A large capacity fuel cell generation apparatus is often composed of a plurality of fuel cells connected in parallel. Since each fuel cell has different properties, a reverse current will occur owing to property differences if fuel cells are simply connected in parallel.
FIG. 1 shows an example, in which two fuel cells 1A and 1B are connected in parallel to form a fuel cell generation apparatus which supplies power to a load 2.
FIG. 2 shows voltage-current characteristics of the fuel cells 1A and 1B. They exhibit proper drooping characteristics A and B, and have proper no-load open-circuit voltages E.sub.OA and E.sub.OB when the current is zero. In FIG. 2, the no-load open-circuit voltage E.sub.OB of the fuel cell 1B is lower than the no-load open-circuit voltage E.sub.OA of the fuel cell 1A. The voltage-current characteristic of the fuel cell generation apparatus consisting of the two fuel cells 1A and 1B is represented by a line S which is obtained by combining the current values of the two fuel cells at identical voltages. For example, when the output voltage of the fuel cell generation apparatus is e.sub.s, the output current i.sub.s of the fuel cell generation apparatus is the sum of a current i.sub.A due to the fuel cell 1A and a current i.sub.B due to the fuel cell 1B. In FIG. 2, i.sub.2 designates the output current of the fuel cell 1A when the output voltage of the fuel cell generation apparatus is E.sub.OB. When the load current i.sub.s is lower than the current i.sub.2, the current of the fuel cell i.sub.B becomes negative, which means that a reverse current will flow into the fuel cell 1B. In other words, if the two fuel cells 1A and 1B have different no-load open-circuit voltages E.sub.OA and E.sub.OB, and E.sub.OA &gt;E.sub.OB, a reverse current will flow into the fuel cell 1B having a lower no-load open-circuit voltage, when the output voltage of the fuel cell generation apparatus e.sub.s is in a range of E.sub.OB &lt;e.sub.s &lt;E.sub.OA. Since the steady state operation of the fuel cell generation apparatus is performed at the output current i.sub.s which is much greater than i.sub.2, the reverse current does not occur. However, since the output current i.sub.s increases from zero at the start of the operation, the reverse current flows until the output current i.sub.s reaches the current i.sub.2.
Although the foregoing description handles the most simple case where two fuel cells are connected in parallel, a common fuel cell generation apparatus includes n (n is an integer greater than two) fuel cells connected in parallel. Assuming that no-load open-circuit voltages of the n fuel cells are E.sub.O1, E.sub.O2, E.sub.O3, . . . , E.sub.Oi, . . . , E.sub.On, and that E.sub.O1 &gt;E.sub.O2 &gt;E.sub.O3 &gt;. . . E.sub.Oi &gt;. . . &gt;E.sub.On, forward currents will flow through all the n fuel cells when the output voltage e.sub.s is lower than any no-load open-circuit voltage E.sub.Oi, and hence, no reverse current will occur. However, when the output voltage e.sub.s is E.sub.Oi-1 &gt;e.sub.s &gt;E.sub.Oi, although forward currents will flow through the fuel cells whose no-load open-circuit voltages are E.sub.O1, E.sub.O2, . . . E.sub.Oi-1, reverse currents will flow through the fuel cells whose no-load open-circuit voltages are E.sub.Oi, E.sub.Oi+1, . . . , E.sub.On.
Such reverse currents will not only increase power loss, but also degrade the characteristics of the fuel cells. Accordingly, it is essential for a fuel cell generation apparatus including fuel cells connected in parallel to prevent a reverse current from flowing.
FIG. 3 shows a conventional fuel cell generation apparatus, in which two fuel cells are connected in parallel and a reverse current is prevented. The fuel cells 1A and 1B are connected in series with diodes 3A and 3B, respectively, and the serial circuits of the fuel cell and the diode are connected in parallel to form a fuel cell generation apparatus. The diodes 3A and 3B prevent the reverse current from flowing even when the output voltage is higher than the minimum no-load open-circuit voltage.
In this arrangement, however, since the output currents of individual fuel cells flow through the diodes, the diodes must have current capacity corresponding to a maximum load current. Thus, when large capacity fuel cells are connected in parallel through the large capacity diodes corresponding to the capacity of the fuel cells, there arises a problem in that the fuel cell generation apparatus becomes expensive, or technically difficult to be implemented depending on capacity. In addition, with the arrangement of FIG. 3, another problem arises in that the efficiency of the fuel cell generation apparatus reduces owing to the loss of the diodes because the output current flows through the diodes without fail.